1. Field of the Invention
The present invention generally relates to testing and replacing defective memory elements through the use of fuses. More specifically, the invention relates to encoding of a bit string in the fuses and to subsequent decoding of the fuses.
2. Description of the Related Art
The customization, test and repair of complex integrated circuits in an automated manufacturing test environment are challenging problems. One issue that arises in this context is the need to automatically repair defective memory elements in large arrays of memory on integrated circuit chips. In the past, metal fuses have been used for such repair. More recently, electrically programmed fuses (“e-fuses”) have been developed.
E-fuses are currently manufactured as polysilicon links. Such an e-fuse has two intended logic states. To obtain a logical “1” state, an e-fuse is “programmed” (also referred to as blown, opened, open circuited, etc.), such as by applying a pulse of 10 mA for 200 microseconds. This current disturbs the polysilicon link, dramatically increasing its resistance. For a relatively lower resistance, logical “0” state, an e-fuse is not subjected to such a “programming” current, and, thus, the polysilicon link is left intact. The logical state of an e-fuse can generally be detected by a sense circuit.
E-fuses are significantly smaller than metal fuses. This is good, because as devices decrease in size, it becomes possible to put more devices onto an integrated circuit chip. Thus, the trend is to larger and larger memory arrays. Because e-fuses have fewer mechanical dependencies, they can even shrink in size as device technology and fabrication processes develop. Consequently, more and more e-fuses are being included on chips to repair increasing numbers of defective memory elements.
One difficulty with e-fuses is that their programming is not entirely reliable. That is, sometimes a programming current does not change the resistance of an e-fuse as much as intended, so that the supposedly programmed e-fuse does not clearly appear to have been programmed when sensed by a sense circuit. This unreliability presents a particularly significant problem, because this kind of failure in the programming of even a single e-fuse on a chip, if uncorrectable, may spoil the chip.